The present invention relates generally to semiconductor thermal processing systems, and more specifically to a system and an apparatus for rapid thermal processing using a segmented cold plate for cooling a semiconductor substrate.
High temperature processing of silicon wafers is important for manufacturing modern microelectronics devices. Such processes, including silicide formation, implant anneals, oxidation, diffusion drive-in and chemical vapor deposition (CVD), may be performed at high temperatures using conventional thermal processing techniques. Furthermore, many microelectronics circuits require feature sizes smaller than one micron and junction depths less than a few hundred angstroms. In order to limit both the lateral and downward diffusion of dopants, as well as to provide a greater degree of control during processing, it is desirable to minimize the duration of high temperature processing.
Semiconductor wafers, flat panel displays and other similar substrates typically have numerous material layers deposited thereon during device fabrication. Some commonly deposited layers (e.g., spin-on glass (SOG) films) may contain contaminants, defects or undesirable microstructures that can be reduced in number or altogether removed by heating or xe2x80x9cannealingxe2x80x9d the substrate at an appropriate temperature for an appropriate time. Other deposited layers (e.g., copper films) may have properties that undesirably change over time or xe2x80x9cself-annealxe2x80x9d, resulting in unpredictable deposited layer properties (e.g., unpredictable resistivity, stress, grain size, hardness, etc.). As with contaminants, defects, and undesirable microstructures, deposited layer properties often can be stabilized by a controlled annealing step. Following the annealing step, the substrate preferably is rapidly cooled to stop the annealing process, and so that other processes can be performed on the substrate, in order to increase throughput.
Conventionally, annealing is performed within a quartz furnace that must be slowly pre-heated to a desired annealing temperature, or within a rapid thermal process (RTP) system that can be rapidly heated to a desired annealing temperature. Thereafter, an annealed substrate is transferred to a separate cooling module that conventionally employs a cooled substrate support and is slightly backfilled with a gas such as helium to enhance thermal conduction. The separate cooling module increases equipment cost and complexity, as well as equipment footprint, and decreases substrate throughput by requiring undesirable substrate transfer time between the heating and cooling systems.
Another approach for minimizing processing time utilizes a heat treatment apparatus such as a single-wafer RTP system. Single-wafer rapid thermal processing of semiconductor wafers provides a powerful and versatile technique for fabrication of very-large-scale-integrated (VLSI) and ultra-large-scale-integrated (ULSI) electronic devices. There are several challenges, however, to meeting the thermal requirements of rapid thermal processing. For example, fast rates of change of wafer temperature are typically desired, as well as temperature uniformity across the wafer during the temperature changes.
According to one single-wafer lamp-based RTP system of the prior art, for example, the wafer is vertically translated between a position proximate to a heating lamp and a position proximate to a solid cold plate. When the wafer is translated from the position proximate to the heating lamp to the position proximate to the cold plate, rapid temperature changes occur during what is termed xe2x80x9cramp-downxe2x80x9d, wherein the temperature of the wafer desirably decreases rapidly to a predetermined temperature. The temperature variation across the wafer during ramp-down, however, should be sufficiently uniform during cooling in order to prevent damage to the wafer such as warpage or cracking. Accordingly, a temperature profile of the cold plate should be substantially uniform across the surface thereof.
According to the exemplary prior art lamp-based RTP system, several translating pins are utilized in order to translate the wafer between the cold plate and the heating lamp, wherein the pins vertically translate through holes in the cold plate, thereby supporting the wafer. In a lamp-based RTP system, however, a distance which the wafer travels between the heat lamp and the cold plate is minimal, since thermal radiation transmitted by the heat lamp can be substantially limited by, for example, cutting power to the heat lamp, or by shielding the wafer by a shutter. Therefore, since the distance of travel between the heating lamp and the cold plate is relatively short, the pins are typically relatively short in length. Consequently, the pins may have significantly small diameters, whereby linear integrity over the short length of the pins can be achieved with the relatively small diameters. Having small diameter pins further minimizes a dimension of the holes in the cold plate, through which the pins translate. Limiting the dimension of the holes in the cold plate is of great concern, since the size of the holes in the cold plate plays a significant role in thermal uniformity across the cold plate during ramp-down. Accordingly, larger holes that may accommodate larger diameter pins may have detrimental effects on the thermal uniformity across the cold plate, and hence, can lead to detrimental effects on the temperature uniformity of the wafer during cooling.
As illustrated in FIG. 1, a simplified lamp-based RTP system 10 of the prior art is depicted, wherein the system comprises a heater lamp 20, a wafer holder assembly 30 and a single-piece cold plate 40. The wafer holder assembly 30 further comprises pins 50 that support the wafer 60, wherein the pins are operable to translate through holes 70 in the cold plate 40 to permit the wafer 60 to be translated between the heater lamp 20 and the cold plate. Typically, a distance between the heater lamp 20 and the cold plate 40 is small (e.g., less than an 20 mm), as discussed above, wherein the diameter of the pins 50 and the holes 70 are respectively small.
The system 10 of the prior art, however may suffer detrimental effects if the distance between the heater lamp 20 and the cold plate 40 is significantly increased. Vertical furnaces, for example, typically translate a wafer over a large distance (e.g., 300 mm or more) within a heater chamber, thereby typically requiring a more robust wafer holder assembly. Furthermore, the substantially solid cold plate 40 utilized in the prior art cannot typically be utilized in a vertical furnace where backside temperature sensing during heating of the wafer is performed. Since a significantly large-diameter hollow elevator shaft (e.g., 10 mm diameter) and accompanying temperature sensing equipment is typically utilized in a vertical furnace RTP chamber, a significantly large hole in the sole cold plate would be needed to accommodate the elevator. Such a large hole would adversely affect the thermal uniformity across the cold plate, and thus, the temperature uniformity across the wafer during cooling.
The conventional cold plate 40 of the prior art may further comprise a plurality of gas ports (not shown) in the cold plate 40 to generally provide a cooling gas to the wafer 60, whereby cooling of the wafer in the molecular flow regime can occur. Cooling in the molecular regime is generally caused by a flowing of gas molecules over a distance which is less than the mean free path for these molecules. However, cooling of the wafer 60 in the molecular flow regime generally limits an ability to fine-tune a temperature profile across the wafer 60.
Therefore, for at least the above-mentioned reasons, an improved cold plate for a vertical furnace RTP system and a method for cooling a wafer is needed to alleviate many of the problems associated with the prior art.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to a semiconductor thermal processing system and apparatus for thermally cooling a semiconductor substrate. The thermal processing system is operable to efficiently cool a substrate in a generally uniform manner across the substrate, thereby improving process control. According to one aspect of the present invention, a semiconductor thermal processing system and associated apparatus are disclosed which provide efficient and uniform cooling by a segmented cold plate.
A thermal process chamber is disclosed, wherein a substrate holder residing within the thermal process chamber is operable to vertically transport a semiconductor substrate between a heating position proximate to a heater assembly and a cooling position proximate to the cold plate. In one example, the cold plate comprises a plurality of radially-moveable segments, wherein the plurality of segments are operably coupled to a radial translation device. The radial translation device is operable to radially translate each of the plurality of segments between an engaged position and a disengaged position, wherein the engaged position generally defines a contiguous surface of the cold plate, and wherein the plurality of segments generally contact with one another. In the disengaged position, a plurality of gaps are defined between each of the plurality of segments when the plurality of segments are radially extended.
According to another aspect of the invention, an elevator operably coupled to the substrate holder is operable to vertically translate the substrate between the heating position and cooling position, wherein the plurality of segments of the cold plate are in the engaged position when the substrate is in the cooling position. Furthermore, when the substrate is moved from the cooling position to the heating position, the plurality of segments are operable to move into the disengaged position, such that the substrate holder can pass through the plurality of gaps between the plurality of segments. A controller is furthermore disclosed, wherein the controller is operable to control a vertical position of the substrate holder and a radial position of the plurality of segments.
According to yet another exemplary aspect of the invention, the cold plate comprises a plurality of holes adapted to allow one or more gases to flow through the cold plate in order to selectively allow thermal conduction in the viscous regime between the substrate and the cold plate. According to still another aspect, the thermal processing system further comprises one or more temperature sensors for detecting one or more temperatures associated with one or more regions of the substrate. A tunable gas source is also disclosed, wherein the tunable gas source is operable to selectively supply one or more gases to the plurality of holes in the cold plate, wherein an amount of the one or more gases supplied to the cold plate is controlled by the controller, and wherein the control is based, at least in part, on the one or more measured temperatures.
According to another exemplary aspect of the present invention, a method for cooling a substrate in a thermal processing system is disclosed, wherein the thermal processing system comprises a segmented cold plate. The method comprises lowering a substrate holder to a cooling position, wherein the substrate holder passes though one or more gaps between a plurality of segments of the cold plate, and wherein the substrate is brought within close proximity the cold plate. The segments of the cold plate are subsequently moved into an engaged position, wherein the segments move radially inward, thereby defining a generally contiguous cold plate. The substrate is then cooled by thermal conduction of heat from the substrate to the cold plate.
According to still another exemplary aspect of the invention, one or more temperatures associated with one or more regions of the substrate are measured, and one or more gases are flowed through holes in the cold plate, whereby thermal conduction in the molecular regime between the substrate and the cold plate can be augmented. An optimum mixture of the one or more gases can be determined based, at least in part, on the one or more measured temperatures, and an amount of the one or more gases can be modified to reflect the optimum mixture for each of the one or more regions.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.